Porous composite and manufacturing method thereof

ABSTRACT

Provided are a porous composite expressed by Chemical Formula 1 and having a porosity of 5% to 90%, and a method of preparing the same: 
       MO x   &lt;Chemical Formula 1&gt;
         where M and x are the same as described in the specification.       

     According to the present invention, since a molar ratio (x) of oxygen to a molar ratio of (semi) metal in the porous composite is controlled, an initial efficiency of a secondary battery may be increased. Also, since the porous composite satisfies the above porosity, a thickness change rate of an electrode generated during charge and discharge of the secondary battery may be decreased and lifetime characteristics may be improved.

TECHNICAL FIELD

The present invention relates to a porous composite and a manufacturingmethod thereof, and more particularly, to a porous composite expressedby MO_(x) (where 0.5<x<1) and having a porosity of 5% to 90%, and amanufacturing method thereof.

BACKGROUND ART

Recently, lithium secondary batteries have received the most attentiondue to their characteristics of high energy density and long lifetime.In general, a lithium secondary battery includes an anode formed of acarbon material or a lithium metal alloy, a cathode formed of lithiummetal oxide, and an electrolyte in which a lithium salt is dissolved inan organic solvent.

Lithium has been initially used as an anode active material constitutingthe anode of the lithium secondary battery. However, since lithium mayhave low reversibility and safety, a carbon material has currently beenmainly used as the anode active material of the lithium secondarybattery. Although the carbon material may have a lower capacity thanlithium, the carbon material may have smaller volume changes as well asexcellent reversibility and may also be advantageous in terms of cost.

Recently, the demand for high-capacity lithium secondary batteries hasgradually increased as the use of the lithium secondary battery hasexpanded. As a result, a high-capacity electrode active materialreplaceable with a low-capacity carbon material has been required. Forthis, research into using a (semi) metal, such as silicon (Si) and tin(Sn), which exhibits a higher charge and discharge capacity than acarbon material and is electrochemically alloyable with lithium, as anelectrode active material has been undertaken.

In a case where the (semi) metal electrochemically alloyable withlithium, such as silicon, is used, cracks or fine particles may begenerated due to the changes in volume caused by repeated charge anddischarge of a battery, and thus, the battery may degrade. As a result,capacity of the battery may decrease. Also, there has been a case inwhich oxide of the (semi) metal was typically used as an electrodeactive material in order to reduce the cracks or fine particles due tothe changes in volume. However, in this case, since lithium oxide orlithium metal oxide, as an irreversible phase caused by an initialreaction with lithium ions, is formed, an initial efficiency of thebattery may decrease.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides a porous composite, in which an initialefficiency may be improved by controlling a molar ratio of oxygencombined with (semi) metal, and lifetime characteristics may be improvedand a thickness change rate during charge and discharge may be decreasedby including a plurality of pores on a surface or the surface and insideof the porous composite, and a method of preparing the same.

Technical Solution

According to an aspect of the present invention, there is provided aporous composite expressed by Chemical Formula 1 and having a porosityof 5% to 90%:

MO_(x)  <Chemical Formula 1>

where M is at least one element selected from the group consisting ofsilicon (Si), tin (Sn), aluminum (Al), antimony (Sb), bismuth (Bi),arsenic (As), germanium (Ge), lead (Pb), zinc (Zn), cadmium (Cd), indium(In), titanium (Ti), and gallium (Ga), and 0.5<x<1.

According to another aspect of the present invention, there is provideda method of preparing a porous composite including mechanical alloyingafter mixing (semi) metal particles and (semi) metal oxide particles;contacting the alloyed mixed particles with the electrodeposit solutionmade by mixing of a fluorinated solution and a metal precursor solutionto electrodeposit metal particles on the surface of the mixed particles;etching the mixed particles having the metal particles electrodepositedthereon by contacting the mixed particles with an etching solution; andcontacting the etched mixed particles with a metal removing solution toremove the electrodeposited metal particles.

According to another aspect of the present invention, there is providedan anode active material including the porous composite.

According to another aspect of the present invention, there is providedan anode including the anode active material.

According to another aspect of the present invention, there is provideda lithium secondary battery including the anode.

Advantageous Effects

According to the present invention, since a molar ratio (x) of oxygen toa molar ratio of (semi) metal in a porous composite expressed byChemical Formula 1 may be controlled to be greater than 0.5 and lessthan 1 by using mechanical alloying, an initial efficiency of asecondary battery may be increased. Also, since the porous compositeincludes a plurality of pores on a surface or the surface and insidethereof and satisfies the above-described porosity, a thickness changerate of an electrode generated during charge and discharge of thesecondary battery may be decreased and lifetime characteristics may beimproved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating a porous composite according toan embodiment of the present invention (black parts: (semi) metal, whiteparts: (semi) metal oxide); and

FIG. 2 is a scanning electron microscope (SEM) image of the porouscomposite according to the embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, the present invention will be described in more detail toallow for a clearer understanding of the present invention.

It will be understood that words or terms used in the specification andclaims shall not be interpreted as the meaning defined in commonly useddictionaries. It will be further understood that the words or termsshould be interpreted as having a meaning that is consistent with theirmeaning in the context of the relevant art and the technical idea of theinvention, based on the principle that an inventor may properly definethe meaning of the words or terms to best explain the invention.

A porous composite according to an embodiment of the present inventionmay be expressed by the following Chemical Formula 1 and may have aporosity of 5% to 90%.

MO_(x)  <Chemical Formula 1>

where M is at least one element selected from the group consisting ofsilicon (Si), tin (Sn), aluminum (Al), antimony (Sb), bismuth (Bi),arsenic (As), germanium (Ge), lead (Pb), zinc (Zn), cadmium (Cd), indium(In), titanium (Ti), and gallium (Ga), and 0.5<x<1.

A porosity of the porous composite according to the embodiment of thepresent invention is in a range of 5% to 90%, may be in a range of 20%to 80%, and for example, may be in a range of 30% to 70%.

In the case that the porosity of the porous composite is less than 5%,volume expansion during charge and discharge may not be prevented. Inthe case in which the porosity of the porous composite is greater than90%, mechanical strength may be decreased due to a plurality of poresincluded in the porous composite, and thus, the porous composite may befractured during manufacturing processes (slurry mixing, pressing aftercoating, etc.) of a battery.

Herein, the porosity may be defined as follows:

Porosity=volume of pores per unit mass/(specific volume+volume of poresper unit mass)

The measurement of the porosity is not particularly limited. Accordingto an embodiment of the present invention, the porosity, for example,may be measured by a Brunauer-Emmett-Teller (BET) method or mercury (Hg)porosimetry.

Also, a BET specific surface area of the porous composite may be in arange of 2 m²/g to 100 m²/g.

In the case that the specific surface area is greater than 100 m²/g, aside reaction with an electrolyte solution may be difficult to becontrolled due to the wide specific surface area. In the case in whichthe specific surface area is less than 2 m²/g, sufficient pores may notbe formed, and thus, the volume expansion during the charge anddischarge of a lithium secondary battery may not be effectivelyaccommodated.

The specific surface area of the porous composite may be measured by aBET method. For example, the specific surface area may be measured by a6-point BET method according to a nitrogen gas adsorption-flow methodusing a porosimetry analyzer (Belsorp-II mini by Bell Japan Inc.).

With respect to the porous composite according to the embodiment of thepresent invention, pores may be formed on a surface or the surface andinside of the composite.

A diameter of the pore on the surface of the porous composite may be ina range of 10 nm to 1,000 nm. In the case that the diameter of the poreis less than 10 nm, pores may be clogged due to the volume expansioncaused by the charge and discharge. In the case in which the diameter ofthe pore is greater than 1,000 nm, cracks may occur around the pores inthe porous composite due to the pores having a size relatively largerthan a diameter of the porous composite. As described above, since theporous composite according to the embodiment of the present inventionmay include the plurality of pores on the surface or the surface andinside of the composite and may satisfy the above porosity, thethickness change rate of the electrode generated during the charge anddischarge of the secondary battery may be decreased and the lifetimecharacteristics may be improved.

Also, the porous composite according to the embodiment of the presentinvention may include (semi) metal particles and oxide particles of(semi) metal.

In particular, since a molar ratio of oxygen to a molar ratio of (semi)metal in the porous composite, i.e., x in Chemical Formula 1, may becontrolled to be greater than 0.5 and less than 1, for example, in arange of 0.6 to 0.9, an initial efficiency of the secondary battery maybe increased.

The molar ratio (x) of oxygen may be measured by an amount of oxygenthat is included in a gas generated by heating the porous composite andmay be measured with a commercial oxygen analyzer. In the case that themolar ratio is 0.5 or less, the initial efficiency may be high but theamount of oxygen that may inhibit the volume expansion may be low, andthus, the lifetime and the inhibition of thickness expansion may bereduced even if a porous structure is formed. In the case in which themolar ratio is 1 or more, the amount of oxygen may increase, and thus,the initial efficiency may decrease.

Specifically, the porous composite according to the embodiment of thepresent invention may have a structure in which the (semi) metalparticles are surrounded by the (semi) metal oxide particles. Therefore,it may be estimated that the volume change of the (semi) metal particlesduring the charge and discharge of the secondary battery may beinhibited by the (semi) metal oxide particles.

The (semi) metal is not particularly limited so long as it is a metal orsemimetal that is alloyable with lithium. Non-limiting examples of the(semi) metal may be a semimetal or metal selected from the groupconsisting of Si, Sn, Al, Sb, Bi, As, Ge, Pb, Zn, Cd, In, Ti, Ga, and analloy thereof.

The (semi) metal oxide is a metal oxide or oxide of semimetal and is notparticularly limited so long as it is an oxide of the metal or semimetalthat is alloyable with lithium. Non-limiting examples of the (semi)metal oxide may be an oxide of the semimetal or metal selected from thegroup consisting of Si, Sn, Al, Sb, Bi, As, Ge, Pb, Zn, Cd, In, Ti, Ga,and an alloy thereof.

In the porous composite according to the embodiment of the presentinvention, the (semi) metal particles may be Si, the (semi) metal oxideparticles may be SiO₂, and M in Chemical Formula 1 may be Si. Since theamount of SiO₂ may decrease and the amount of oxygen that may becombined with Si may decrease as the amount of Si increases, the porouscomposite may be expressed by SiO_(x) (where 0.5<x<1). Also, the (semi)metal particles and the (semi) metal oxide particles in the porouscomposite may be in the form of nanocrystals. A diameter of thenanocrystals may be in a range of 0.1 nm to 100 nm.

The porous composite is formed by mixing the (semi) metal particles andthe (semi) metal oxide particles by mechanical alloying, and in thiscase, a molar ratio of the (semi) metal to the (semi) metal oxide may bein a range of 80:20 to 50:50. However, the present invention is notlimited thereto. In the case that the molar ratio is less than the aboverange, since reversible capacity may be low, an effect as ahigh-capacity electrode active material may be insignificant. Also,since an amount of lithium oxide or lithium metal oxide formed as anirreversible phase due to an initial reaction with lithium ions may beexcessively high, the initial efficiency may decrease. In contrast, inthe case in which the molar ratio is greater than the above range, sincevolume changes of the (semi) metal generated during the charge anddischarge of the battery may not be sufficiently inhibited, cracks orfine particles of the electrode active material may occur. Thus, thecapacity and lifetime of the battery may be reduced.

Also, with respect to the porous composite, the molar ratio (x) ofoxygen to the molar ratio of the (semi) metal in the porous compositemay be controlled to be greater than 0.5 and less than 1 by mechanicalalloying. Therefore, since a reaction of lithium and the (semi) metaloxide or a reaction of lithium and oxygen may decrease during the chargeand discharge of the battery when the porous composite is used as anelectrode active material, the amount of the formed lithium oxide orlithium metal oxide, i.e., the irreversible phase, may decrease. Thus,the initial efficiency of the battery may be improved. In addition,volume changes of the electrode active material generated during thecharge and discharge of the battery may be inhibited.

Furthermore, since the volume changes of the (semi) metal particles maybe inhibited or minimized by the (semi) metal oxide particles, thevolume changes of the electrode active material may be inhibited orminimized during the charge and discharge of the battery. Thus, asecondary battery using the porous composite according to the embodimentof the present invention may have a lower volume change rate based on aninitial value than a typical secondary battery using a (semi) metalalloyable with lithium. According to an embodiment of the presentinvention, the typical secondary battery using the (semi) metalalloyable with lithium may have a volume change of theoretically 400%,substantially about 700% to about 800%. In contrast, the volume changerate based on the initial value of the secondary battery using theporous composite according to the embodiment of the present inventionmay be about 100%.

The porous composite according to the embodiment of the presentinvention may further include a carbon coating layer on the porouscomposite in order to improve a battery performance of the secondarybattery.

The carbon coating layer may be formed by a heat treatment method afterpitch or a hydrocarbon-based material is mixed, or a chemical vapordeposition (CVD) method.

In addition, the porous composite may be used by being mixed with anadditional carbon material, and specifically, the carbon material mayinclude natural graphite, artificial graphite, or mesocarbon microbeads(MCMB).

Also, the present invention may provide a method of preparing a porouscomposite including mechanical alloying after mixing (semi) metalparticles and (semi) metal oxide particles, contacting the alloyed mixedparticles with the electrodeposit solution made by mixing of afluorinated solution and a metal precursor solution to electrodepositmetal particles on the surface of the mixed particles, etching the mixedparticles having the metal particles electrodeposited thereon bycontacting the mixed particles with an etching solution, and contactingthe etched mixed particles with a metal removing solution to remove theelectrodeposited metal particles.

The method of preparing a porous composite according to an embodiment ofthe present invention may include mechanical alloying after mixing(semi) metal particles and (semi) metal oxide particles.

Herein, the expression “mechanical alloying” is referred to as making amixed composite having a uniform composition by applying a mechanicalforce. The mechanical alloying may be performed by using amechano-fusion apparatus that is known in the art. Examples of themechano-fusion apparatus may be a high-energy ball mill, a planetaryball mill, a stirred ball mill, or a vibrating mill. Among the aboveapparatuses, the mechanical alloying may be performed with thehigh-energy ball mill. However, the present invention is not limitedthereto. For example, (semi) metal having an average particle diameterof about 2 μm to about 10 μm, for example, about 2 μm to about 5 μm and(semi) metal oxide having an average particle diameter of about 2 μm toabout 10 μm, for example, about 2 μm to about 5 μm are mixed and themixture is put in a mechano-fusion apparatus, such as a ball mill, withballs having a diameter of about 5 mm. Then, mechanical alloying isperformed at a rotational speed of about 300 rpm to about 3,000 rpm atroom temperature.

As a result, the mixture of the (semi) metal (black parts) and the(semi) metal oxide (white parts) are ground due to high-energy ballmilling to become fine powder, and thus, the mixture may be uniformlymixed and simultaneously, a mixed composite may be formed (see FIG. 1).

However, in order to more efficiently control the molar ratio (x) ofoxygen to the molar ratio of the (semi) metal in the mixed compositeformed by the preparing method according to the embodiment of thepresent invention to be greater than 0.5 and less than 1, the mixing andmechanical alloying of the (semi) metal particles and the (semi) metaloxide particles may be performed in an atmosphere in which any contactwith oxygen is avoided. For example, the mixing and mechanical alloyingmay be performed in an inert atmosphere including nitrogen gas, argongas, helium gas, krypton gas, or xenon gas, a hydrogen gas atmosphere,or a vacuum atmosphere.

In this case, the average particle diameters of the (semi) metalparticles and the (semi) metal oxide particles are not particularlylimited. However, small diameters may be used to reduce mixing time andmechanical alloying treatment time.

Also, a weight ratio of the mixture of the (semi) metal particles andthe (semi) metal oxide particles to the balls may be in a range of 1:10to 1:20. In the case that the weight ratio is out of the above range,compressive stress may not be transferred to the mixture, or since amore than necessary amount of balls are used, it may be inefficient interms of energy efficiency.

A stainless ball or zirconia ball having a diameter of 0.1 mm to 10 mmmay be used as the ball.

A method of preparing a porous composite according to an embodiment ofthe present invention may include contacting the alloyed mixed particleswith the electrodeposit solution made by mixing of a fluorinatedsolution and a metal precursor solution to electrodeposit metalparticles on the surface of the mixed particles.

In this case, the mixed particles emit electrons due to the fluorinatedsolution and metal ions in the solution receive electrons to be reducedand electrodeposited on the surfaces of the mixed particles. Once themetal particles are electrodeposited on the surfaces of the mixedparticles, continuous electrodeposition may occur as the metal particleitself becomes a catalyst site.

The fluorinated solution used may be one or more selected from the groupconsisting of hydrogen fluoride (HF), silicon fluoride (H₂SiF₆), andammonium fluoride (NH₄F), and the metal precursor solution may includeone or more selected from the group consisting of silver (Ag), gold(Au), platinum (Pt), and copper (Cu). The fluorinated solution and themetal precursor solution may be mixed at a volume ratio ranging from10:90 to 90:10. In the case that the volume ratio of the fluorinatedsolution included is less than 10, an amount of the metal precursorformed on the surfaces of the mixed particles may be small and areaction rate may be very slow, and thus, a preparation time mayincrease. In the case in which the volume ratio of the fluorinatedsolution included is greater than 90, formation speed of the metalprecursor may be very fast, and thus, uniform and small-sized metalparticles may not be electrodeposited on the surfaces of the mixedparticles.

Also, an amount of the metal particles electrodeposited on the mixedparticles may be controlled according to a concentration of thefluorinated solution and a contact time of the mixed particles with themetal precursor solution. An amount of the contacted mixed particles maybe in a range of 0.001 parts by weight to 50 parts by weight based on100 parts by weight of a mixed solution of the fluorinated solution andthe metal precursor solution.

The method of preparing a porous composite according to the embodimentof the present invention includes etching the mixed particles havingmetal particles electrodeposited thereon by contacting the mixedparticles with an etching solution. Nanopores, mesopores, and macroporesare formed in the mixed particles through the etching process.

The metal precursor ionized in a HF solution is reduced in the form ofmetal particles and electrodeposited on the surfaces of the mixedparticles, the mixed particles are continuously dissolved whiletransferring electrons to the metal particles, and simultaneously, thereduction of metal ions occurs in the electrodeposited metal particles.According to the foregoing method, the mixed particles in contact withthe metal particles may be continuously etched to form a porousstructure having a honeycomb shape at least on the surface thereof.

A mixed solution of HF solution and ethanol (C₂H₅OH) may be used as theetching solution, and in some cases, hydrogen peroxide (H₂O₂) may beadded. An amount of the HF solution may vary according to a degree ofetching. However, the HF solution and the C₂H₂OH solution may be mixedat a volume ratio ranging from 10:90 to 90:10, and the H₂O₂ solution maybe mixed at a volume ratio of 10 to 90 based on the mixed solution ofthe HF solution and the C₂H₅OH solution. At this time, H₂O₂ may act as acatalyst and may control the amount of the electrodeposited metalparticles or the degree of etching according to the concentrationthereof.

Also, the etching may be performed for 30 minutes to 12 hours. In thecase that the etching is performed less than 30 minutes, the formationof pores in the mixed particles may be insignificant. In the case inwhich the etching is performed greater than 12 hours, the mixedparticles are excessively etched, and thus, mechanical properties of themixed particles may be deteriorated.

The method of preparing a porous composite according to the embodimentof the present invention includes removing the electrodeposited metalparticles by contacting the etched mixed particles with a metal removingsolution, and may prepare particles, in which pores in a honeycomb shapeare formed at least on the surfaces of the mixed particles.

The metal removing solution used may be one or more selected from thegroup consisting of nitric acid (HNO₃), sulfuric acid (H₂SO₄), andhydrochloric acid (HCl).

In the method of preparing a porous composite according to theembodiment of the present invention, the etching method after themechanical alloying may form pores without changing the crystalstructure of the mixed composite.

Furthermore, the method of preparing a porous composite according to theembodiment of the present invention may further include mixing theporous composite with a carbon precursor after removing theelectrodeposited metal particles and then heat treating the mixture tocoat the surface of the porous composite with carbon.

Any carbon precursor may be used without limitation so long as it mayform carbon by a heat treatment, and for example, graphite, pitch or ahydrocarbon-based material may be used. Examples of thehydrocarbon-based material may be any one selected from the groupconsisting of furfuryl alcohol, glucose, sucrose, a phenol-based resin,a phenol-based oligomer, a resorcinol-based resin, a resorcinol-basedoligomer, a phloroglucinol-based resin, a phloroglucinol-based oligomer,and an unsaturated hydrocarbon gas, such as ethylene, propylene, oracetylene, or a mixture of two or more thereof.

According to an embodiment of the present invention, the carbonprecursor may be used in an amount ranging from 1 wt % to 30 wt % basedon a total weight of the porous composite.

In the case that the amount of the carbon precursor used is less than 1wt %, a uniform coating layer may not be formed, and thus, electricalconductivity may decrease. In the case in which the amount of the carbonprecursor used is greater than 30 wt %, an additional irreversiblereaction may occur, and thus, the capacity and initial efficiency may bedecreased.

Also, the heat treatment, for example, may be performed in a temperaturerange of 300° C. to 1,400° C.

Also, the present invention may provide an anode active materialincluding the porous composite.

The anode active material according to an embodiment of the presentinvention may further include a carbon-based material. That is, theporous material may be used by being mixed with a typically usedcarbon-based material.

The typically used carbon-based material may be one or more selectedfrom the group consisting of natural graphite, artificial graphite,MCMB, carbon fibers, and carbon black.

Furthermore, the present invention provides a secondary batteryincluding a cathode including a cathode active material; a separator; ananode including the anode active material; and an electrolyte.

Since the secondary battery according to an embodiment of the presentinvention may include an anode active material including the porouscomposite, the initial efficiency of the secondary battery may beimproved.

For example, the anode may be prepared by coating an anode currentcollector with a mixture of an anode active material, a conductiveagent, and a binder, and then drying the coated anode current collector.If necessary, a filler may be further added. The cathode may also beprepared by coating a cathode current collector with a cathode activematerial and drying the coated cathode current collector.

The separator is disposed between the cathode and the anode, and a thininsulating film having high ion permeability and mechanical strength maybe used as the separator. Since the current collectors, electrode activematerials, conductive agent, binder, filler, separator, electrolyte, andlithium salt are known in the art, the detailed descriptions thereof areomitted in the present specification.

The separator is disposed between the cathode and the anode to form abattery structure, the battery structure is wound or folded to put in acylindrical battery case or prismatic battery case, and then a secondarybattery is completed when the electrolyte is injected thereinto. Also,the battery structure is stacked in a bi-cell structure, impregnatedwith the electrolyte, and a secondary battery is then completed when theproduct thus obtained is put in a pouch and sealed.

Hereinafter, the present invention will be described in detail,according to specific examples. The invention may, however, be embodiedin many different forms and should not be construed as being limited tothe embodiments set forth herein.

Example 1 1-1. Preparation of SiO_(x)

Si having an average particle diameter of about 2 μm to about 5 μm andSiO₂ having an average particle diameter of about 2 μm to about 5 μmwere mixed at a molar ratio of 70:30 to form a mixture. The mixture andstainless steel balls having a diameter of about 5 mm were mixed at aweight ratio of 1:15, and mechanical alloying was then performed at arotational speed of about 1,000 rpm for 180 minutes by using ahigh-energy ball mill to prepare SiO_(x). The preparation of the mixedcomposite was performed in an argon atmosphere and x of the SiO_(x) was0.6.

1-2. Electrodeposition of Ag on Surfaces of SiO_(x) Particles

300 ml of a solution having 10% hydrogen fluoride (HF) and 300 ml of asolution having 10 mM silver nitrate (AgNO₃) were mixed for 10 minutes.2 g of SiO_(x) was added to the solution having hydrogen fluoride andsilver nitrate mixed therein and the solution was mixed for 5 minutes,and then SiO_(x) having Ag electrodeposited thereon was prepared byfiltering, washing, and drying the mixture.

1-3. Chemical Etching

200 ml of a solution having 5% hydrogen fluoride and 100 ml of asolution having 1.5 wt % hydrogen peroxide (H₂O₂) added thereto weremixed for 10 minutes. SiO_(x) having Ag particles electrodepositedthereon was added to the etching solution having hydrogen fluoride andhydrogen peroxide mixed therein and mixed for 30 minutes. Then, porousSiO_(x) was prepared by filtering, washing, and drying the mixture.

1-4. Ag Removal

100 ml of 60% nitric acid (HNO₃) was heated to 50° C. and the porousSiO_(x) was then added thereto and mixed for 2 hours. Porous SiO_(x) foran anode active material having Ag removed therefrom was prepared byfiltering, washing, and drying the mixture.

Example 2

SiO_(x) was prepared in the same manner as in Example 1 except that Siand SiO₂ were mixed at a molar ratio of 50:50. In this case, x was 0.9.

Examples 3 and 4

Porous SiOx prepared in Examples 1 and 2 were respectively put in arotary tube furnace and argon gas was introduced at a flow rate of 0.5L/minute. Then, the temperature was increased to 1,000° C. at a heatingrate of 5° C./minute. Porous SiO_(x) coated with a conductive carbonmaterial were prepared by performing a heat treatment for 3 hours whilerotating the rotary tube furnace at a speed of 10 rpm and respectivelyflowing argon gas and acetylene gas at a rate of 1.8 L/minute and 0.3L/minute. In this case, an amount of the conductive carbon material was5 wt % of the porous SiO_(x).

Examples 5 to 8 Preparation of Secondary Battery

SiO_(x) prepared in Examples 1 to 4 respectively as anode activematerials, acetylene black as a conductive agent, and polyvinylidenefluoride as a binder were mixed at a weight ratio of 85:5:10 and themixture was mixed with a N-methyl-2-pyrrolidone solvent to prepare aslurry. One surface of a copper current collector was coated with theprepared slurry to a thickness of 65 μm, dried and rolled. Then an anodewas prepared by punching into a predetermined size.

LiPF₆ was added to a non-aqueous electrolyte solvent prepared by mixingethylene carbonate and diethyl carbonate at a volume ratio of 30:70 toprepare a 1 M LiPF₆ non-aqueous electrolyte solution.

A lithium foil was used as a counter electrode, a polyolefin separatorwas disposed between both electrodes, and a coin-type secondary batterywas then prepared by injecting the electrolyte solution.

Comparative Example 1

A secondary battery was prepared in the same manner as in Example 5except that commercial silicon monoxide (SiO) was used as an anodeactive material.

Comparative Example 2

Commercial silicon monoxide was coated with a carbon material in thesame manner as in Example 3, and a secondary battery was prepared in thesame manner as in Example 5.

Comparative Example 3

Pores were formed in commercial silicon monoxide, the commercial siliconmonoxide was then coated with a carbon material in the same manner as inExample 3, and a secondary battery was prepared in the same manner as inExample 5.

Comparative Example 4

SiO_(0.6) was prepared in the same manner as in Example 1-1, and asecondary battery was prepared in the same manner as in Example 5 exceptthat SiO_(0.6) was used as an anode active material.

The following Table 1 represents porosities and BET specific surfaceareas of porous composites or SiO_(x) prepared in Examples 1 to 4 andComparative Examples 1 to 4.

TABLE 1 BET specific Anode active Porosity surface area Examplesmaterial (%) (m²/g) Example 1 SiO_(0.6) 38 43.7 Example 2 SiO_(0.9) 3843.7 Example 3 SiO_(0.6)/C 38 43.7 Example 4 SiO_(0.9)/C 38 43.7Comparative SiO 0 2.3 Example 1 Comparative SiO 38 43.7 Example 2Comparative SiO/C 38 43.7 Example 3 Comparative SiO_(0.6) 0 2.3 Example4

Experimental Example 1 Scanning Electron Microscope (SEM) Analysis

SEM analysis was performed on a surface of the anode active materialprepared in Example 1, and the results thereof are presented in FIG. 2.

As illustrated in FIG. 2, it may be understood that a plurality of poreswere included on the surface or the surface and inside of the porouscomposite.

Experimental Example 2 Initial Efficiency, Lifetime Characteristics, andThickness Change Rate Analysis

The following experiments were performed in order to investigate initialefficiencies, lifetime characteristics, and thickness change rates ofthe secondary batteries prepared in Examples 5 to 8 and ComparativeExamples 1 to 4.

First cycle charge capacity and first cycle discharge capacity weremeasured to obtain a ratio of the first cycle discharge capacity to thefirst cycle charge capacity for each battery.

Lifetime characteristics of each battery were measured by performingcharge and discharge at 0.5 C after a third cycle and the lifetimecharacteristics were represented as a ratio of discharge capacity in a49th cycle to the first cycle discharge capacity.

Each secondary battery was disassembled in a charge state of a 50thcycle and a thickness change rate was calculated by measuring differencein thicknesses of an electrode after the 50th cycle and before a chargecycle.

The following Table 2 presents initial efficiencies, lifetimecharacteristics, and thickness change rates of the secondary batteriesprepared in Examples 5 to 8 and Comparative Examples 1 to 4.

TABLE 2 Discharge capacity (mAh/g, Initial Lifetime Thickness dischargedefficiency character- change rate Examples at 1.5 V) (%) istics (%) (%)Example 5 1720 82.3 90 95 Example 6 1650 74.5 93 85 Example 7 1720 82.393 90 Example 8 1650 74.5 96 80 Comparative 1568 72 75 190 Example 1Comparative 1568 72 85 120 Example 2 Comparative 1568 72 88 110 Example3 Comparative 1720 82.3 70 200 Example 4 Initial efficiency: (firstcycle discharge capacity/first cycle charge capacity) × 100 Lifetimecharacteristics: (discharge capacity in a 49th cycle/first cycledischarge capacity) × 100 Thickness change rate: (electrode thicknessafter a 50th cycle − electrode thickness before a cycle)/electrodethickness before the cycle × 100

As illustrated in Table 2, it may be understood that the dischargecapacities, initial efficiencies, and lifetime characteristics of thesecondary batteries prepared in Examples 5 to 8 were improved incomparison to those of the secondary batteries prepared in ComparativeExamples 1 to 4. It may be also understood that the thickness changerates of the secondary batteries prepared in Examples 5 to 8 weresignificantly lower than those of the secondary batteries prepared inComparative Examples 1 to 4.

Specifically, the secondary batteries prepared in Examples 5 to 8, inwhich anode active materials including SiO_(x) (where 0.5<x<1) wereused, exhibited a discharge capacity of 1650 mAh/g or more, an initialefficiency of 74.5% or more, lifetime characteristics of 93% or more,and a thickness change rate of 95% or less. When compared with thesecondary batteries prepared in Comparative Examples 1 to 4 in whichanode active materials including SiO_(x) (where x=1) were used, thedifferences in discharge capacities, initial efficiencies, lifetimecharacteristics, and thickness change rates were about 100 mAh/g, about10% or more, 10% to 20% or more, and a maximum of about 100%,respectively.

Also, anode active materials having a molar ratio of oxygen of 0.6,i.e., SiO_(x) (where x=0.6), were used in Example and ComparativeExample 4. When Example 5 which used porous SiO_(x) having a porosity of38% was compared with Comparative Example 4 which used SiO_(x) having aporosity of 0%, the discharge capacities and initial efficiencies werethe same, but there were significant differences in their lifetimecharacteristics and thickness change rates. In particular, the thicknesschange rate of Example 5 was 95%, but Comparative Example 4 exhibited athickness change rate of 200%.

The reason for this is that since the plurality of pores were includedon the surface or the surface and inside of the porous composite and theporosity was in a range of 5% to 90%, the thickness change rate of theelectrode generated during the charge and discharge of the secondarybattery may be reduced, and as a result, the lifetime characteristicsmay be improved.

INDUSTRIAL APPLICABILITY

Since a porous composite of the present invention may increase aninitial efficiency of a secondary battery, may decrease a thicknesschange rate of an electrode generated during charge and discharge, andmay improve lifetime characteristics, the porous composite may besuitable for a secondary battery.

1. A porous composite expressed by Chemical Formula 1 and having aporosity of 5% to 90%:MO_(x)  <Chemical Formula 1> where M is at least one element selectedfrom the group consisting of silicon (Si), tin (Sn), aluminum (Al),antimony (Sb), bismuth (Bi), arsenic (As), germanium (Ge), lead (Pb),zinc (Zn), cadmium (Cd), indium (In), titanium (Ti), and gallium (Ga),and 0.5<x<1.
 2. The porous composite of claim 1, wherein a porosity ofthe porous composite is in a range of 20% to 80%.
 3. The porouscomposite of claim 1, wherein x in Chemical Formula 1 is measured by anamount of oxygen that is included in a gas generated by heating theporous composite.
 4. The porous composite of claim 1, wherein the porouscomposite comprises (semi) metal and oxide of the (semi) metal, whereinthe (semi) metal is selected from the group consisting of Si, Sn, Al,Sb, Bi, As, Ge, Pb, Zn, Cd, In, Ti, Ga, and an alloy thereof.
 5. Theporous composite of claim 4, wherein the (semi) metal is Si, the (semi)metal oxide is SiO_(x), and M is Si.
 6. The porous composite of claim 1,wherein pores are formed on a surface or the surface and inside of theporous composite.
 7. The porous composite of claim 6, wherein a diameterof the pore on the surface of the porous composite is in a range of 10nm to 1,000 nm.
 8. The porous composite of claim 1, wherein aBrunauer-Emmett-Teller (BET) specific surface area of the porouscomposite is in a range of 2 m²/g to 100 m²/g.
 9. The porous compositeof claim 4, wherein the (semi) metal and the (semi) metal oxide are in aform of nanocrystals.
 10. The porous composite of claim 9, wherein adiameter of the nanocrystals is in a range of 0.1 nm to 100 nm.
 11. Theporous composite of claim 1, further comprising a carbon coating layeron the porous composite.
 12. A method of preparing a porous composite,the method comprising: mechanical alloying after mixing (semi) metalparticles and (semi) metal oxide particles; contacting the alloyed mixedparticles with the electrodeposit solution made by mixing of afluorinated solution and a metal precursor solution to electrodepositmetal particles on the surface of the mixed particles; etching the mixedparticles having the metal particles electrodeposited thereon bycontacting the mixed particles with an etching solution; and contactingthe etched mixed particles with a metal removing solution to remove theelectrodeposited metal particles.
 13. The method of claim 12, whereinthe (semi) metal is selected from the group consisting of silicon (Si),tin (Sn), aluminum (Al), antimony (Sb), bismuth (Bi), arsenic (As),germanium (Ge), lead (Pb), zinc (Zn), cadmium (Cd), indium (In),titanium (Ti), and gallium (Ga), and an alloy thereof.
 14. The method ofclaim 12, wherein a molar ratio of the (semi) metal particles to the(semi) metal oxide particles is in a range of 80:20 to 50:50.
 15. Themethod of claim 12, wherein the mechanical alloying after the mixing ofthe (semi) metal particles and the (semi) metal oxide particles isperformed in an atmosphere in which oxygen is blocked.
 16. The method ofclaim 15, wherein the oxygen-blocking atmosphere is an inert atmospherecomprising nitrogen gas, argon gas, helium gas, krypton gas, or xenongas, a hydrogen gas atmosphere, or a vacuum atmosphere.
 17. The methodof claim 12, wherein the mechanical alloying is selected from the groupconsisting of high-energy ball milling, planetary ball milling, stirredball milling, and vibrating milling.
 18. The method of claim 12, whereinthe fluorinated solution is one or more selected from the groupconsisting of hydrogen fluoride (HF), silicon fluoride (H₂SiF₆), andammonium fluoride (NH₄F).
 19. The method of claim 12, wherein the metalprecursor solution comprises one or more selected from the groupconsisting of silver (Ag), gold (Au), platinum (Pt), and copper (Cu).20. The method of claim 12, wherein the etching solution is a mixedsolution of HF and ethanol (C₂H₅OH).
 21. The method of claim 20, whereinthe etching solution further comprises hydrogen peroxide (H₂O₂).
 22. Themethod of claim 12, wherein the metal removing solution is one or moreselected from the group consisting of nitric acid (HNO₃), sulfuric acid(H₂SO₄), and hydrochloric acid (HCl).
 23. The method of claim 12,further comprising mixing the porous composite with a carbon precursorafter removing the electrodeposited metal particles and then heattreating the mixture to coat a surface of the porous composite withcarbon.
 24. The method of claim 23, wherein the carbon precursor isgraphite, pitch or a hydrocarbon-based material.
 25. The method of claim23, wherein the carbon precursor is used in an amount ranging from 1 wt% to 30 wt % based on a total weight of the porous composite.
 26. Themethod of claim 23, wherein the heat treatment is performed in atemperature range of 300° C. to 1,400° C.
 27. An anode active materialcomprising the porous composite of claim
 1. 28. An anode comprising theanode active material of claim
 27. 29. A lithium secondary batterycomprising the anode of claim 28.